1. Field of the Invention
This invention relates to a beam position detecting device which is provided for a device such as a laser printer using a laser beam.
2. Background of the Invention
A beam position detecting device of this type has been disclosed by Japanese Patent Application (OPI) No. 5227/1984 (the term "OPI" as used herein means "an unexamined published application"). The arrangement of the device will be briefly described with reference to FIG. 1. A laser beam outputted by a laser (not shown) is reflected by a rotating polygonal mirror 11, and the laser beam thus reflected reaches the surface of a photosensitive drum (not shown) with its local scanning speed compensated by an f-.theta. lens 12. A mirror 14 for reflecting the laser beam towards a beam sensor 13 is disposed in the path of the laser beam to detect whether or not the laser beam is at a predetermined scanning position. The mirror 14 is mounted on a vertical plate 16 of a mirror holder 15. The angle of the mirror 14 with respect to a horizontal plane can be adjusted by turning a screw 18 which is engaged with a horizontal plate 17 which is extended horizontally from the vertical plate 16. The lower end of the screw 18 is abutted against a base plate 19. Therefore, as the screw 18 is turned, the vertical plate 16 is tilted, as a result of which the angle of the mirror 14 with respect to the base plate (horizontal plane) 19 is changed.
However, it is impossible to adjust the angle of the mirror 14 within the horizontal plane because the mirror holder 15 has been fixedly secured to the base plate 19 with two screws 21 and the lower end of the screw 18 is kept abutted against the base plate 19. Therefore, if the angle of the mirror 14 in the horizontal plane is to be changed, then an optical system (encircled with the broken line in FIG. 2 for instance) including the rotating polygonal mirror 11, the f-.theta. lens 12, the mirror 14 and the beam sensor 13 must be displaced, in its entirety, in the scanning direction to detect the correct scanning position of the laser beam.